Factors affecting fatigue wear resistance of gray cast iron

Rolling fatigue contact is a very complex problem, so there are many factors affecting the fatigue wear resistance of gray cast iron, but it can be roughly divided into the following points:

(1) Loading load

Load is one of the main reasons affecting fatigue wear life. It is generally believed that the service life of gray cast iron is inversely proportional to the cube of load, that is:

Where n is the cycle at failure, P is the applied load, and C is a constant.

(2) Microstructure of gray cast iron

① Gas and non-metallic inclusions in gray cast iron. The nitrogen atoms and oxygen atoms in the middle gap of gray cast iron can fix dislocations and accelerate the initiation of fatigue cracks. Therefore, the higher the content of nitrogen and oxygen, the worse the resistance to contact fatigue wear of gray cast iron. In addition, inclusions in gray cast iron, such as sulfide and brittle oxide, will destroy the continuity of gray cast iron surface and have a serious adverse effect on fatigue wear.

② Heat treatment organization. Network carbide, excess retained austenite and coarse martensite will reduce the contact fatigue wear resistance of gray cast iron. This is mainly because the network carbide or coarse martensite will reduce the strength of the phase interface, which is conducive to the fatigue crack propagation along the interface. However, an appropriate amount of retained austenite can absorb some energy by plastic deformation or transformation into martensite in fatigue contact, so as to improve the fatigue wear resistance of gray cast iron. The normal heat treatment structure should be fine martensite matrix with uniform, fine and dispersed carbides and a certain amount of residual austenite. In this way, the surface of gray cast iron has high cutting tension to prevent or slow down the sliding of crystals along the crystal surface.

(3) Surface quality

① Surface roughness. The change of roughness means the change of the actual contact area between the contact bodies. Under the premise of certain loading load, large contact stress will be generated on the contact surface with high roughness, which is easy to cause the collapse of local gray cast iron on the contact surface and accelerate the formation of fatigue defects. On the contrary, the smaller the surface roughness, the worse the sensitivity to stress concentration, so the better the fatigue wear resistance of gray cast iron.

② Surface burns. Grey cast iron will produce instantaneous high temperature during the pre-processing process and change the structure, which will reduce the strength of grey cast iron and reduce the fatigue limit of grey cast iron.

③ Surface corrosion. Due to the effect of environment, chemical reaction will occur on the surface of gray cast iron, and even pits will be formed on the surface in serious cases, so as to accelerate the formation of contact fatigue defects.

(4) Residual stress

The influence of residual stress on rolling contact fatigue was proposed by Zaretsky in 1965, and its relationship is as follows:

Due to the residual compressive stress, the maximum shear stress on the surface of the contact surface can be reduced( τ Max ⁡) r, so as to improve the contact fatigue life of gray cast iron.

(5) Lubrication conditions

Different lubricants have different effects on the fatigue wear resistance of gray cast iron. Increasing the viscosity of lubricating oil can improve the fatigue wear resistance; The different chemical composition of lubricating oil also has a certain impact on the fatigue life: when the lubricant contains oxygen and water, the contact fatigue life of gray cast iron will be seriously reduced; When the chemical composition that has corrosive effect on the crack tip is contained, it is conducive to crack propagation, so as to significantly reduce the fatigue life. If additives can form strong surface film and reduce friction, it will be beneficial to the improvement of fatigue wear resistance.

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